Bar codes, RFID tags, and other wireless identification and communication schemes have been widely used to identify individual merchandise or other objects. RFID tags and other wireless identification networks have also been proposed or used for tracking and controlling inventory as it enters and leaves a store or warehouse. They have also been used for tracking inventory as it moves from one location to another within a production line. Recently the electronic product code (EPC) has been adopted for RFID applications. EPC is a 92 bit ID code which allows specific objects to be identified.
Wireless sensor nodes have been proposed or used for monitoring conditions such as temperature, humidity, displacement, acceleration, and strain in structures, such as bridges, roads, vehicles, buildings, and rotating parts, as described in commonly assigned patent application Ser. No. 09/731,066, “Data Collection and Storage Device” to Christopher P. Townsend, et al.; U.S. Pat. No. 6,529,127, “System for Remote Powering and Communication with a Network of Addressable Multichannel Sensing Modules,” to Seven W. Arms, et al.; U.S. patent application Ser. No. 10/668,827, “Remotely Powered and Remotely Interrogated Wireless Digital Sensor Telemetry System,” to Michael John Harnel, et al.; U.S. patent application Ser. No. 10/379,223, “Energy Harvesting for Wireless Sensor Operation and Data Transmission,” to Michael John Hamel, et al.; and U.S. patent application Ser. No. 10/769,642, “Shaft Mounted Energy Harvesting for Wireless Sensor Operation and Data Transmission,” to Steven Arms, et al., all of which are incorporated herein by reference. The 1024-041 application shows the idea of a network of addressable sensors. The 115-008 and 115-014 applications show energy harvesting systems for addressable wireless sensor nodes. Wireless sensor nodes have also been proposed or used for monitoring vehicles and industrial equipment to provide condition based maintenance.
However, all of these systems have treated each bar code, RFID tag, or wireless sensor node as an individual entity. None of the systems so far proposed have been satisfactory in efficiently providing information about which objects of a large collection of potentially disconnected objects are grouped together in some fashion to form a subgroup. A bar code scanner or a token has been needed to identify members of a group. But none of these methods allow automatic grouping. For example, none of the previous systems has been able to efficiently and automatically provide information about which objects remain in the same shipping container and distinguish from other nearby objects that are not in that container and from other objects that have been removed from the container. Nor have previous systems been able to automatically detect which of many machine parts are attached to a machine. Thus, a better system for detecting subgroups of wireless sensor nodes that share some common feature is needed, and this solution is provided by the following.